How do you put signs on the ground in a raid?

Here is what I am talking about:

When I am tanking for my guild, I always have to ask someone to do it, because it does not work for me from the slide-in from the standard raid interface. Guild told me it works via the slide in, but it does not work for me. I am always Raid Leader or Assistant so I should have the rights to do that.

How exactly do I put those signs down? It would be really handy for my future tanking endeavours.

These are world marks (or also "raid beacons"). You have to be promoted in order to place them while member of a raid. You can find the option to set them when expanding the raid-control (per default on the left side of the screen)

It is also possible to place them with a macro/console command:

/worldmarker 1

/clearworldmarker 1

'1' is an example here, you can use 1-8 for the different symbols. The beacon should occur at your mousecursorposition then.

Further information and a howto for macros can be found here